This invention relates to procedures and apparatus for the continuous casting of metals.
More particularly, the invention is directed to continuous casting operations of the type wherein the metal being cast is progressively advanced through a chilled and open-ended mold while undergoing peripheral solidification to provide a solid outer shell for the emerging cast body, which typically has a molten core extending for some distance beyond the mold. Additional positive cooling is commonly applied to the body beyond the mold to promote solidification of the core. In the casting operations with which the present invention is most specifically concerned, molten metal is progressively supplied to the mold (as casting proceeds) from a tundish or the like through a so-called submerged shroud, i.e. a conduit or tube that has discharge ports submerged beneath the molten metal level in the mold.
A familiar example of such operations, to which detailed reference will be made herein for purposes of illustration, is the casting of steel billets in an axially vertical mold having a coaxially disposed shroud projecting downwardly into the mold.
In these and other casting operations, inclusions such as oxide particles are unavoidably present in the delivered molten metal. Desirably, the inclusions thus introduced are carried into the slag layer floating on the molten metal surface in the mold during casting. It is found, however, that even though the inclusions are lower in specific gravity than the metal, a proportion of them tend to become entrapped at the solid-liquid interface within the mold. Consequently, the cast billet may contain significant quantities of these inclusions, especially in its outer portion, which corresponds to the locus of the solidification front at the region within the mold where the entrapment occurs. Material from the slag layer may also be entrapped as inclusions in the outer portion of the billet. Apart from the general undesirability of incorporating contaminant matter in a cast billet, the occurrence of such inclusions, e.g. alumina, presents a serious specific problem in that it interferes with machinability of the billet, since the outer portion of the billet is commonly subjected to machining.
Stated in other words, it would be very desirable to minimize or prevent entrapment of inclusions, whether from the introduced flow of molten metal or from the slag layer, at the solid-liquid interface of a body being cast within a continuous casting mold.
Various expedients have heretofore been suggested for dealing with these and other problems associated with conventional continuous casting practice. For instance, it has been proposed to provide a shroud having lateral discharge ports oriented to direct the flow of supplied molten metal obliquely upwardly within a mold. In operations not employing a shroud, it has also been proposed to deliver molten metal to the mold within a surrounding sleeve of inert gas, or to create, by electromagnetically produced molten metal circulation, an upward flow of metal adjacent the periphery of an axially vertical mold. These expedients, however, have not afforded wholly satisfactory reduction of inclusions in the outer portion of cast billets, or have suffered from other disadvantages. In particular, it has been difficult to produce fluid circulation electromagnetically within a mold, because the coils through which electric current is passed to cause such circulation must be disposed externally of the electrically conductive wall of the mold, although (as described in U.S. Pat. No. 3,693,697, issued Sept. 26, 1972 to Alexander A. Tzavaras, one of the applicants herein) effective circulation can be produced electromagnetically in the molten core of the portion of a billet that projects beyond a mold, for control of solidification conditions within that portion.